Siloxane elastomers



United States Patent 3,185,963 SILQXANE ELASTOMERS John Tudor Lewis, Dinas Powis, Glamorgan, and Nigel Page, arr-y, Glamorgan, Wales, assignors to Midland Silicones Limited, London, England No Drawing. Filed Aug. 25, Eel, Set. No. 133,781 Claims priority, application Great Britain, Sept. 5, 1969,

30,569 3 Claims. (Cl. 269-465) This invention relates to improvements in the curing of organopolysiloxane elastomeric compositions.

Room temperature vnlcanizing (RTV) silicone elastomers are now well known commercial products and are used in a wire variety of applications varying from encapsulating of electric components to the caulking of voids in many types of structure and equipment.

Probably the best known of these RTV elastomers are those which rely for their room temperature vulcanization and curing properties on the presence of two ingredients, namely, a metal salt of a carboxylic acid such as dibutyl tin dilaurate and an alkyl silicate or polysilicate. In order to prevent premature vulcanization of the elastomer it is convenient to supply the elastomer as a two component system, one part consisting of a simple mixture of the metal salt and an alkyl silicate and/ or a polysilicate and the other component comprising the organopolysiloxane together with any other required fillers or additives. In use the two components of the system are brought to gether and vulcanization of the elastorneric composition takes place.

Such a system has, for the most part, proved satisfactory but it suffers from several disadvantages. For example, one of the preferred metal carboxylates is dibutyl tin dilaurate and a simple mixture of 30 parts of this salt with 70 parts by weight of ethylorthosilicate has a freezing point which is high enough to preclude its use as a liquid catalyst at low temperatures.

A more serious disadvantage of the previously known mixtures of metal-carboxylate and silicate or polysilicate is the great variation of activity shown by the catalyst system, not only when prepared from ditierent batches of each of the catalyst components but even when mixtures are made up from samples taken from the same batches. When, as is often the case, it is necessary to prepare several batches of the catalyst mixture from a large batch of each of the two ingredients, this variation in activity of the catalyst makes it impossible to arrive at a fixed catalyst dose for curing the elastomer under a given set of conditions. Moreover, further difficulties are encountered because of the tendency of the catalyst to vary in activity during periods of storage or transit.

It is the primary object of this invention to introduce a catalyst system for use in room temperature vulcanizing silicone elastomer stocks which will exhibit a high degree of consistency in activity. A novel and improved catalyst system for silicone elastomers is a further object. Other objects and advantages of this invention are detailed in or will be apparent from the disclosure and claims following.

The present invention provides an improved catalyst sys tern for vulcanizing organopolysiloxane rubber stocks capable of vulcanization at room temperature consisting essentially of the product obtained by heating to a temperature in the range 80 to 200 C. for at least minutes a tin salt of a carboxylic acid and a silicate selected from alkyl silicates of the general formula R Si(OR') where R is an alkyl radical, R is an alkyl radical containing less than 6 carbon atoms and a is O or 1 and partial hydrolysis products of said alkyl silicates.

The present invention further provides novel silicone rubber stocks which consist essentially of (l) a hydroxylcontaining organopolysiloxane having a viscosity of at Too least 100 cs. at 25 C. said organopolysiloxane having the average uni-t formula R nSiO T where each R" is a monovalent hydrocarbon radical or halogenated monovalent hydrocarbon radical and n has an average value of from 1.90 to 2.10 inclusive, and (2) a catalyst system for vulcanizing the siloxane prepared by heating together a tin salt of a carboxylic acid and an alkyl silicate of the general formula R Si(OR) where each R is an alkyl radical and each R is an alkyl radical containing less than 6 carbon atoms and a=0 or 1 or a partial hydrolysis product of said alkyl silicate, at a temperature in the range to 200 C. for at least 15 minutes.

The alkylsilicates employed in the preparation of the catalyst system of this invention include alkylorthosilicates of the formula Si(OR) and alkyltrialkoxysilanes of the formula RSi(OR) wherein R is an alkyl radical and R is an alkyl radical of less than 6 carbon atoms. Examples of operative alkylsilicates are ethyltrimethoxysilane, methylbutoxydiethoxysilane, propyltripr-opoxysilane, methyltriethoxysilane, ethyltriethoxysilane, ethylorthosilicate, and n-bu=tylorthosilicate. Also operative are alkyl polysilicates which can be prepared by the partial hydrolysis of th above defined alkylsilicates and can be represented by the formula where R and R are as above defined, a has an average value from 0 through -1.0, b has an average value of at least 1.0 and a+b is less than 4. Examples of such polysilicates are ethyl polysilicate, isopropylpolysilicate, n-butyl polysilicate, dimethyltetraethoxydisiloxane, trimethyl pentabutoxytrisiloxane,

where Et is ethyl, Me is methyl and x is 5-25, and a methyl endblocked dimethoxysiloxane or diethoxysiloxane. Mixtures of (1) monomeric alkylsilicates, (2) one or more monomeric alkylsilicates and one or more polymeric alkyl-polysilicates, and (3) polymeric alkylpolysilicates can be employed.

As the second reactant for producing the improved catalysts of the present invention there are employed the carboxylic acid salts of tin, for example, dibutyl tin dilaurate, tributyl tin monolaurate, dibutyl tin diacetate, tin naphthenate, tin oleate, d-ibutyl tin dimaleate and stannons octoate.

The vulcanizable organopolysiloxanes employed in the present invention are polymeric materials of the average unit formula T where each R" is a monovalent radical selected from hydrocarbon .and halogenohydrocarbon radicals and n has an average value of 1.9 to 2.1, and containing silicon bonded -OH radicals. These polymers contain predominantly (i.e., at least mol percent) diorganosiloxane uni-ts (R" SiO) but can contain up to 10 mol percent of monoorganosiloxane units and/or triorganosiloxane units. The substituent organic groups represented by R" can be any monovalent hydrocarbon radical or halogenated monovalent hydrocarbon radical. For example, the R" groups can be alkyl groups such as methyl, ethyl and octadecyl; cycloaliphatic groups such as cyclohexyl and cyclopentyl; alkenyl groups such as vinyl and allyl; aromatic hydrocarbon radicals such as phcnyl, xenyland naphthyl and halogenated monovalent hydrocarbon radicals such as chloromethyl, bromophenyl, 3,3,3-triiluoropropyl, trifluorovinyl and hexafluoroxylyl; The vulcanizable siloxane polymers are fully disclosed and described in US. Patents No. 2,843,555 and No. 2,983,694 as well as in Australian Patent No. 216,878.

Preparation of the improved catalysts of the present in- EXAMPLE 2 A portion of the refluxed catalyst mixture containing 58% by weight each of ethyl polysilicate and dibutyl tin dilaurate was taken and mixed with an equal volume of 5 r vention is accomplished by heating together the organoftraethyl olthoslhcaie- T1115 catijllyst f f to a silicon compound and the tin carboxylate. It is most con- Y endblocked dImQthWPOIYSHOXaHQhquld P venient to heat the two components of the catalyst mixture portlonspf by Welght of the polysiloxane liquid. together'for a period of about minutes to 5 hours and The time Faken the hqmd a minutes at a temperature of 80 to 200 C. The preferred time 10 cpmpared n ni when 115mg the Same P P and ,temperature are minutes at tron of the catalyst consistlng of the unrefluxed mixture of Another feature of this invention is the increased efiecdibuty} U11 dl-allfate d yl pols silicate, and 16 minutes fiveness as vulcanizing agents of the novel catalyst when the refluxed catalyst mlxtu re was used undiluted. positions of the present invention. As a result of this 5 increased activity it is now possible to dilute the prepared 15 EXAMPLE 3 catalyst composition with the organosilicon compound Example 2 w repeated employing a refluxed mi q Obtam catalyst equalflctlvlty g ture of 50% by Weight of each of ethyl orthosilicate and ta{mng a Smaller PTPPOTUPH the m1 Salt than a 51mph dibutyl tin dilaurate. This mixture was diluted with an riate we1 to sniconon e met rou s. inbefore described the improved catalyst compositions of I 3 m g the dimathyl polysfloxgnegfiqllfid this invention provide room temperature vulcanizing siloxwas 12 minutes V ane rubbers with physical properties at least as good as That which is Claimed is: those previouslylmown in the art. is to be understood L A process for Obtaining a composition of matter m addltlon to the .0rganpO1.y.sfloXane and the noYel 25 which can be converted at relatively low temperatures to catalysts, the elastominc coiliposmons of the prafsent an elastomeric product comprising forming a mixture of vention can also contain additlves such as fillers, plgments, ingredients consisting esssntiany of (1) a hydroxyl com i f f i g i thi .nvention taining organosiloxane polymer having a viscosity of at o e e as Oman: composl OHS O s 1 1 1st 100 s. at 25 C. and havin the avera e unit formula takes place spontaneously upon mixing the organopoly- 3Q C g g siloxane and the catalyst. Adjustment of the curing rate n can be brought about, within limits, by varying the quan- T tity of catalyst employed.

The following examples, in which the parts are by Where K 1S f i g g ge i g f the Weight, illustrate the invention. group consistingo y rocai on ra re s an a ogeno- I hydrocarbon radicals and n has an average value of from EXAMPLE 1 1.9 to 2.1 inclusive, and (2) the reaction product obtained is h t' t tern eratu ith th 8. 80 t 200 Several mixtures of the two cataiyst compmiems were C 1331 1 55503 of zfi leasti l5 mi ri utes ai tin salt of a fi fi i fi i g i i of the {In i 5;; 4:0 carboxylic acid, and (b) a silicon-containing material I z y 51 f d h d 1 fi was Se as selected from the group consisting of alkylsilicates of the Tlie chz h gn ctisiti of i1; a taly si wi ineasured by genial-a1 formula} RESKORIP 4 a Where each R is an alk'yl b 0 2 k 1 h d v 1 d blo ked radical, each R is an alkyl radical of less than 6 carbon i P t i en y t f atoms and a has an average value of from 0 to 1 inclusive, i z g g ag y zfig g gg fi 2 323: 2 and pfiartial1 hydrolyzates of said alkylsilicates having the a t ormu ture and then employing the mixture which had been pm a heated. a )b During thisseries of experiments the freezing point, or alternatively the temperature at which solid separated was r I also measured. Where R, R and a are as above defined and'b has a value The results obtained are shown in the table in which of at least 1 and less than 4 and the sum of a+b is less the catalyst compositions are given as percentage by than 4. V V I weight, and EPS, DBTDL and E08 represent, ethyl poly- 2. A catalyst system for room temperature vulcanizing silicate, dibutyl tin dilaurate and ethyl orthosilicate resilicone rubbers consisting ofthe reaction product obtained spectively. by heating at a temperature within the range of 80 C. to

Table Catalyst composition P t t V i Before reflux After reflux (iataljyst lioad- O ema 0111 perature, C. tempeiiature, 5512210? C. Freezing Gel time Freezing Gel time polysiloxane EPS DB'IDL EOS point, 0. (activity) point, 0. (activity) fluid, percent;

10 168 95-97 5 50mins 18 1 20 160 91 13 40 mins -12 1 5 30 156-163 95-98 16 30 min 8 1 40 15 88 98 mins 5 0. 5 50 132-153 84 90 mins 4 0.5 132-168 s1 0) 2 hrs... -4 0. 5 125-174 24 hrs.. 4 0. 5 so 115-125 so 24 hrs 4 10 117-126 90 -31 2hrs 40 1 20 80 115-125 88-85 -14 2% hrs... -28 1 30- 70 110-132 92-90 2 2% hrs... --18 1 40 60' 110-125 80 5 =12 hrs 16 0. 5 50 50 110-155 38 9 =12 hrs -10 0.5 00 40 110-145 89-86 11 121mm-.. 7 0.5 70 30 -130 82-84 13 3 hrs., 40 mins... --6 0.5 30 20 105-140 82-79 21 hrs '5 *DBTDL settled out at room temperature (22 C.)

5 5 200 C. for a period of at least 15 minutes (a) a tin salt 3. The catalyst of claim 2 further characterized in that of a carboxylic acid and (b) a silicor1-containing material the tin salt is dibutyl tin dilaurate. selected from the group consisting of alkylsilicates of the general formula R Si(OR') where each R is an alkyl References Cited by the Exammel' radical, each R is an alkyl radical of less than 6 carbon 5 UNITED STATES N S atoms and a has an average value of from 0 to 1 inclusive, and partial hydrolyzates of said alkylsilicates having the gz ii fmmula 2,985,546 5/61 Leavitt 260-465 10 2,999,077 9/61 Nitzsche et a1. 26046.5

where R, R and a are as above defined and b has a value MURRAY TILLMAN Primary E xaminer of at least 1 and less than 4 and the sum of a+b is less MILTON STERMAN, WILLIAM H. SHORT, than 4. Examiners. 

1. A PROCESS FOR OBTAINING A COMPOSITION OF MATTER WHICH CAN BE CONVERTED AT RELATIVELY LOW TEMPERATURES TO AN ELASTOMERIC PRODUCT COMPRISING FORMING A MIXTURE OF INGREDIENTS CONSISTING ESSENTIALLY OF (1) A HYDROXYL CONTAINING ORGANOSILOXANE POLYMER HAVING A VISCOSITY OF AT LEAST 100 CS. AT 25*C. AND HAVING THE AVERAGE UNIT FORMULA 